Numerous natural and synthetic heteropolymeric compounds are known in the art. For example, glycopeptides are commonly found in most eukaryotic cells and on cell membranes. In most cases, glycopeptides are enzymatically and post-translationally synthesized in a cell. Therefore, and from a structural perspective, the backbone of such molecules is a polypeptide with glycosyl side chains covalently attached to pendant R-groups of polypeptide backbone. Likewise, numerous natural polysaccharides are known to carry one or more amino acids. For example, murein (which is a polymer comprising N-acetylglucosamine and N-acetylmuramic acid) is typically derivatized with a pentapeptide that is covalently coupled to the N-acetyl muramic acid in the murein to act as crosslinker between two or more polysaccharide chains. Once again, from a structural perspective, the backbone of such molecules is a polysaccharide to which oligopeptides are covalently bound as pendant groups.
Remarkably, and despite the availability of numerous carbohydrate and amino acid building blocks, heteropolymers in which carbohydrates and peptides form the backbone of a heteropolymer have generally not been reported, with the exception of relatively few and selected molecules. For example, a mixed backbone heteropolymer consisting of DNA and PNA portions was reported in Biochem Pharmacol. 2005 Nov. 1; 70(9):1277-87. In this paper, a sugar-phosphate backbone of two DNA portions was linked to a peptide backbone of a PNA portion. Similarly, certain chimeric molecules in which a PNA (peptide nucleic acid) and lactose form a backbone were described by Zhang et al. in Bioorg Med Chem Lett. 2001 May 21; 11(10):1269-72. While such polymers have desirable properties with respect to nucleic acid hybridization, various drawbacks may remain. Among other things, where the backbone includes a DNA portion, negative charges are typically present at physiological pH due to the phosphate groups in the backbone.
Other functionalized polyester graft copolymers consisting of a linear α-hydroxy-acid polyester backbone having at least one amino acid group incorporated therein and at least one poly(amino acid) side chain extending from an amino acid group in the polyester have been described in U.S. Pat. No. 6,740,310. In still further examples, a polymer of hyaluronic acid and a second polymer of either a non-ionic polymer, a polymeric gum, or a combination thereof are combined to form certain heteropolymers as previously described in U.S. Pat App. No. 2005/0084537. While such polymers may provide certain advantages, controlled and high yield synthesis is often difficult. Moreover, many of such synthetic polymers may elicit an immune response in an animal when implanted.
Therefore, while numerous compositions and methods for heteropolymers are known in the art, all or almost all of them suffer from one or more disadvantages. Most significantly, heteropolymers with mixed backbones in which amino acids and sugars form the backbone in compounds other than PNA-containing compounds are not known. Thus, there is still a need to provide improved heteropolymers, and especially those with an amino acid-carbohydrate mixed backbone.